In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, cellular technologies specified by the third Generation Partnership Program (3GPP) are widely deployed. One evolution into an evolved radio access technology is referred to as Long-Term Evolution (LTE). In LTE, different modes of communication can be used for devices (such as radio access network nodes and wireless terminals) in a cellular network, such as Frequency Division Duplex (FDD), Time Division Duplex (TDD), and half duplex.
In TDD, the uplink (i.e., transmission from wireless terminal to radio access network node) and the downlink (i.e., transmission from radio access network node to wireless terminal) communication between the radio access network node and the wireless terminal use the same frequency band but different time slots to separate the receive (RX) and transmit (TX) instances, i.e. the RX and TX instances take place in different, non-overlapping, time slots.
A typical signal distribution network 100 of a TDD radio structure including baseband circuitry 110a, 110b is shown in FIG. 1. The signal distribution network 100 further comprises a direct up-conversion homodyne transmitter 120, a heterodyne transmitter observation receiver (TOR) 130, a homodyne receiver 140, and radio circuitry 150. Baseband circuitry 110a is configured for TX and baseband circuitry 100b is configured for RX.
The transmitter 120 inter alia comprises digital-to-analogue converters (DACs), an in-phase and quadrature (IQ) modulator 121, and amplifiers, such as power amplifiers (PAs). The transmitter observation receiver 130 inter alia comprises a low-pass filter, a down converter, and an analogue-to-digital converter (ADC) and receives a part of the signal transmitted by the homodyne transmitter 120 by means of a coupler 154. The receiver 140 inter alia comprises a variable gain amplifier 142, an in-phase and quadrature demodulator 141 configured for down-converting the carrier frequency to baseband, low-pass (baseband) filters, and ADCs. Due to RX and TX using the same frequency band in TDD systems, a two ports cavity filter 151 is shared for both receiver 140 and transmitter 120. A circulator 152 is configured to separate the TX path and the RX path. The reflected TX signal from the cavity filter 151 will pass through the switch 153 and be absorbed by termination during the TX time period. By means of the circulator 152 a signal can thus either be directed from the transmitter 120 to the antenna 155 or from the antenna 155 to the receiver 140. During the RX time period the receiver 140 thus receives a reception signal from the switch 153 through a low noise amplifier (LNA) 157 and a band-pass (radio frequency) filter 156.
Homodyne receivers 140 may offer a cost benefit and potential performance advantage over traditional heterodyne receivers. In addition, a homodyne receiver architecture may offer more freedom in addressing multi standard operation and multiple frequency bands of operation using a single hardware module. Homodyne receivers 140 enable eliminating intermediate frequency (IF) stages, thus reducing the number of components compared to heterodyne receivers. Further, directly converting the received signal to effectively zero-IF frequency enables image problems associated with super heterodyne architectures to be ignored.
As noted above, in the homodyne receiver 140, after low-noise amplification, the desired carrier frequency is down-converted to baseband using an IQ demodulator 141. Using a local oscillator (LO), the IQ demodulator 141 generates sum and difference frequencies at the output ports, where low pass filters heavily reject the summation frequency and allow only the difference frequency to pass.
However, some challenges are associated with homodyne receivers 140. One challenge is handling of IQ impairments. IQ amplitude and phase mismatch can cause degraded signal-to-noise (SNR) performance. In an ideal IQ demodulator 141, the baseband IQ signals share a perfect 90° phase relationship between in-phase (I) and quadrature (Q) vectors, and are said to be in perfect quadrature. When IQ signals mismatch, the I and Q vector discrimination will suffer from amplitude and phase errors, which degrade the recovered SNR for the received signals of interest.
Mitigation of IQ impairments may be achieved by proper calibration of the homodyne receiver 140. Some known proposed mechanisms for mitigating IQ impairments in the receiver 140 will be summarized next.
A first group of mechanisms considers the use of prior information, such as pilot tones, about the transmitted signal. This requires calibration of the receiver 140 at startup. As an example, international patent application with publication number WO2014/000807 discloses a receiver device comprising an adaptive IQ correction component arranged to correct IQ errors in an output baseband signal.
A second group of mechanisms additionally considers so called blind compensation, i.e. mechanisms where no prior knowledge about the transmitted signal is available.
A third group of mechanisms considers the use of an auxiliary receiver to provide a reference to the nominal receiver.
Drawbacks of these known proposed mechanisms for mitigating IQ impairments will be summarized next.
The first group of mechanisms needs a pilot tone if calibration is used or a known sequence of data if an a priori solution is chosen. This may lead to delays or a lower transmission throughput due to the need to schedule the known sequence in addition to the payload intended for transmission.
Regarding the use of blind compensation techniques as considered in the second group of mechanisms, such techniques have generally turned out to have poor performance.
The third group of mechanisms requires additional hardware which may occupy a significant printed circuit board size and cost increases as well, especially in multi-branch receiver architectures.
Hence, there is still a need for an improved calibration of a homodyne receiver.